[CPMD-list] thermostat "frequency": mu or omega?

[Lan-Feng Yuan (lyuan@Princeton.EDU)]

  Hi, dear CPMD users,

  We know the vibrational frequencies of H2O molecule are ~ 3800 and 1600 cm-1, then we want to select a thermostat frequency in between, say 2000 cm-1. Then, should I write "NOSE IONS 300. 2000." or "NOSE IONS 300. 1256."? In the latter, I mean the angular frequency omega = 2*pi*mu = 1256 cm-1.

  Why am I asking this? Because when I try to do path integral calculations, I always get information like (number of replicas P=16, T=300 K):

 THERMOSTATS: CHANGE IONIC NOSE FREQUENCIES IN PATH INTEGRAL CASE FOR PRIMITIVE PROPAGATOR
    CHARACTERISTIC FREQUENCY FOR IP=1 :     5234.37 CM**-1 
    CHARACTERISTIC FREQUENCY FOR IP>1 :     5234.37 CM**-1 

  No matter what frequency I select in the input, it is changed to a certain value by the program. The reason is that the coupling between path integral beads is perfectly harmonic and very hard to get thermolized, so CPMD switches the thermostat frequency to the value corresponding to this coupling. That's a good idea. But it seems to me that this value shown is omega rather than mu. This harmonic coupling has the form of 1/2*m*P*(k_B*T/hbar)^2*(r_i-r_i+1)^2, therefore the so-called omega_P=sqrt(P)*k_B*T/hbar. For P=16 and T=300 K,  I get this omega_P=5234 cm-1. It's noteworthy that its frequency mu is ~ 834 cm-1, so it's actually lower than the lowest vibrational mode of water (1600 cm-1), not higher than the highest (3800 cm-1)!

  First, I hope this is not a mistake ... Second, since the "frequency" here is omega, does that mean I should input omega also in other cases?

  Thank you very much! Best regards,

  Lan-Feng

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Dr. Lan-Feng Yuan,
Postdoctoral Associate,
Department of Chemistry,
Princeton University,
Princeton, NJ 08544, USA
Office Phone: 1-609-258-1834
Fax: 1-609-258-6746
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